What an RDR file is and why you might need one

An RDR file is a raw disk image — a complete, byte-for-byte copy of a disk or drive stored as a single file. On macOS, you create one when you need to back up an entire drive, transfer a disk to another computer, or preserve the exact state of a storage device. Unlike a regular folder backup, an RDR file captures everything: the file system structure, permissions, hidden files, and the way data is physically arranged on the disk.

You do not need third-party software to make one. macOS includes the dd command and Disk Utility, both built into every Mac, and either one can create an RDR file from a connected drive or partition.

Key Takeaways

  • macOS Disk Utility can create an RDR file by selecting a drive, choosing Image > Save, and picking the read-only compressed format.
  • The dd command in Terminal creates an RDR file with a single line of code but requires you to identify the correct disk device name first.
  • An RDR file is much larger than a regular backup because it copies every byte of the disk, including empty space.
  • Always unmount the drive before creating an RDR file to avoid corruption or incomplete data capture.
  • Restoring an RDR file back to a drive requires either Disk Utility or the dd command, and will overwrite everything on the target drive.

Using Disk Utility to create an RDR file

Disk Utility is the easiest route if you prefer a graphical interface. Connect the drive or insert the disk you want to copy, then open Disk Utility (in Applications > Utilities). Select the drive in the left sidebar — not a partition within it, but the drive itself. Go to the Image menu at the top and select Save.

A save dialog opens. Name your file, choose a location (an external drive with enough free space is best), and in the Format dropdown select "compressed" or "read-only compressed". Compressed format shrinks the file by removing redundant data, which saves space but takes longer to create. Click Save and wait — the time depends on the drive size and your Mac's speed, but Disk Utility shows progress as it works.

When finished, you have an RDR file (or .dmg file, which is macOS's standard image format and works the same way). You can move this file to another Mac, an external drive, or cloud storage. To restore it later, double-click the file in Finder or use Disk Utility's Restore tab.

Using the dd command in Terminal for more control

The dd command gives you precise control and works faster than Disk Utility for large drives. Open Terminal (Applications > Utilities > Terminal) and type the command, but first you must identify the disk's device name. Type diskutil list and press Enter. You will see a list of all connected drives with names like /dev/disk0, /dev/disk1, and so on. Find the drive you want to copy — match it by size or name — and note its device identifier.

Unmount the drive by typing diskutil unmountDisk /dev/diskX (replace X with the number from the list). Do not eject it; unmounting keeps it connected but stops macOS from using it. Then type the dd command:

sudo dd if=/dev/diskX of=~/Desktop/backup.img bs=4m

Replace /dev/diskX with your actual disk name and ~/Desktop/backup.img with the path and filename you want. The bs=4m part sets the block size to 4 megabytes, which speeds up the copy. Press Enter, type your password when prompted, and wait. Terminal shows no progress bar — it will appear frozen, but it is working. When the command finishes, you see a summary line with the number of blocks copied.

Why RDR files are large and how to manage them

An RDR file is the same size as the entire disk, not just the files on it. If you copy a 500 GB drive that is only half full, the RDR file is still 500 GB because it captures every sector, including the empty space. Compressed format reduces this somewhat, but not by much if the empty space is random data rather than repeated zeros.

Before you create an RDR file, make sure you have enough free space on your destination drive. A 1 TB external drive cannot hold an RDR file from a 1 TB internal drive — you need extra room. If space is tight, consider backing up only the partition you need (select the partition in Disk Utility instead of the whole drive) or using a regular file-based backup instead.

Restoring an RDR file back to a drive

To put an RDR file back onto a drive, you reverse the process. In Disk Utility, select the target drive, go to Image > Restore, choose your RDR file, and click Restore. Disk Utility will overwrite everything on the target drive with the contents of the image.

With dd, unmount the target drive and type:

sudo dd if=~/Desktop/backup.img of=/dev/diskX bs=4m

This writes the image file back to the disk. Again, Terminal shows no progress — wait for the summary line before unplugging the drive. Restoring is slower than creating the image because it must write every byte to the physical disk.

Common mistakes and how to avoid them

The most dangerous mistake is using the wrong device name in dd. If you type /dev/disk0 when you meant /dev/disk1, you will overwrite your Mac's own drive. Always run diskutil list first, match the size and name carefully, and if you are unsure, ask someone to check before you press Enter.

Another mistake is forgetting to unmount the drive before creating the image. If the drive is mounted and actively in use, the RDR file may be incomplete or corrupted. Unmount it first — in Disk Utility, select the drive and click the Unmount button, or in Terminal type diskutil unmountDisk /dev/diskX.

Do not interrupt dd while it is running. If you close Terminal or press Ctrl+C, the image file will be incomplete and useless. Let it finish, even if it takes hours.

When to use RDR files instead of regular backups

An RDR file is best for exact cloning — moving a drive to a new Mac, preserving a disk exactly as it was, or backing up a bootable drive so you can restore it identically later. For everyday backups of documents and photos, a regular backup tool like Time Machine is faster and uses less space.

RDR files are also useful for archiving old drives before you recycle them, or for creating a snapshot of a system before a major change. If you ever need to recover deleted files or investigate a drive's contents in detail, an RDR file lets you mount it as a virtual disk and explore it without touching the original.

Frequently Asked Questions

Can I create an RDR file from a drive that is currently my startup disk?

Yes, but you must boot from a different drive first — either another Mac, a USB recovery drive, or an external drive with macOS installed. You cannot create a complete image of the drive your Mac is currently running from. Restart your Mac and hold Command+R to boot into Recovery Mode, then open Disk Utility from there.

How long does it take to create an RDR file?

It depends on the drive size and speed. A 256 GB drive typically takes 30 minutes to an hour with Disk Utility or dd. Compressed format takes longer because it must analyze the data as it copies. Using an external drive connected via USB 3 is faster than Thunderbolt or older USB versions.

Can I open an RDR file on Windows?

Not directly — Windows does not recognize macOS disk images natively. You would need third-party software like 7-Zip or WinImage to read it. If you need to transfer data between a Mac and Windows, a regular file backup is simpler than an RDR file.

What is the difference between an RDR file and a .dmg file?

They are the same thing — RDR is the file format, and .dmg is the filename extension macOS uses. When you save an image in Disk Utility, it creates a .dmg file, which is an RDR format image. Both terms refer to the same disk image.

Do I need to eject the drive after creating an RDR file?

You should unmount it before creating the image, but after the image is complete, you can eject it normally. Unmounting and ejecting are different — unmount stops macOS from using the drive, while eject safely disconnects it. Always eject external drives before unplugging them.